Oxossi Ayofemi, an Oakland- and San Francisco-based artist, was traveling on BART one day when a pair of turf dancers began to perform in her train car. Most people respond by staring in shock or wonder at the dancers’ contortions, or by looking down so they won’t feel obliged to fish in their pockets for change.But Ayofemi did neither. She watched with curiosity and admiration. Then she spoke to them.The upshot of that communication was an artistic collaboration that went far beyond the BART dancers’ wildest dreams. They became actors in her multimedia exploration of the topic “Black Matter,” in concert with Stanford University astrophysicist Risa Wechsler, at the Contemporary Jewish Museum. And on Aug. 9, they will visit the CJM for the first time as part of a public presentation of the exhibit, which opened July 26.